The second photo is of a rhyolite lava flow in the Firehole Canyon. Rhyolite lavas are extremely viscous, as illustrated nicely by the intensely deformed flow banding in this outcrop – it hasn’t so much flowed, as oozed. This flow occured within the Yellowstone caldera some time after it was excavated by the last big explosive eruption 640,000 years ago.

Perhaps it was pretty obvious in hindsight – but I didn’t expect you all to be all so North America-centric that you wouldn’t guess one of the world’s other geothermal areas. Maybe the pine trees were too much of a giveaway. Regardless, I saw plenty of awesome geology whilst I was away – and I’m planning to share the highlights with you all over the next few weeks.
*incidentally, I may just have a big hole in my mineralogical knowledge (which is entirely possible), but I swear I’ve never heard hydrothermally deposited silica referred to as ‘geyserite’ before. Is this usage limited to North America, perhaps?

Comments (5)

I’ve certainly come across the term geyserite before, but I can’t recall if I have outside the context of Yellowstone. It’s not a term I like very much, as it clearly implies a close relationship with geysers, but gets equally applied to silica-composed tufa-like material.
In the ongoing tension between lumpers and splitters, this is another of the innumerable bits of obscure jargon that I can’t fault anyone for not knowing. I personally prefer the identifier siliceous sinter, which is more encompassing. I guess that would make me a lumper.

Lockwood: I agree that geyserite is a rather misleading term, especially since a substantial fraction of the ‘geyserite’ in Yellowstone is from the flow of hot springs rather than geysers. Sinter is, I seem to recall, the term used in the New Zealand hydrothermal areas – I personally tend to just talk about hyrodthermally deposited silica, because it seems less ambiguous.
LL: I didn’t really get up much beyond the foothills of the Tetons – and I think ‘grabbing’ samples from National Parks is rather frowned upon…

I actually just got back from Yellowstone and the Grand Tetons myself from a 4 day trip over the weekend. I got some really awesome pictures I hope to post shortly. Many of them while I was wearing my “Supervolcano” t-shirt.